We know also that noisome filth spread over a field by the farmer in the autumn or winter loses its offensiveness in a few days, and by the spring neither our eyes nor noses give us any clue to the cause of the fertility of the field which is covered with ordinary 'mould.' This process of 'humification' is largely due to earthworms and other earth dwellers, which pass the earth repeatedly through their bodies, and in doing so reduce it to a very fine powder. I have examined worm castings picked off a lawn, and which, after being slowly dried, have been gently sifted through muslin. Those who have never examined a worm casting in this way will be interested to know of what an impalpable dust the greater part is composed, and will also note the considerable size of the pieces of flint and grit which the animal has used in its living mill, and which have been separated by the muslin sieve. These castings are full of microbes, and those who will take the trouble to scatter the smallest conceivable pinch of this impalpable dust upon a sterilised potato, after the manner and with all the precautions familiar to bacteriologists, will obtain an abundant and varied growth of bacteria and moulds, which will completely baffle their powers of enumeration and discrimination. The greatest hindrance in the bacterial examination of the soil is this _embarras de richesses_, which makes the isolation of different species a matter of extreme difficulty. The bacteria exist in the soil in countless millions, but it must be remembered that they get fewer as we go deeper. The first few inches of the soil are, in the matter of bacterial richness, worth all the rest, and at a depth of five or six feet they appear to be almost non-existent. The practical lesson which we have to lay to heart in applying this knowledge is that the upper layers of the soil are the potent layers in bringing about the circulation of organic matters, and that if we wish to hasten this process we must be careful to place our organic refuse near the surface, and not to bury it deeply, a process by which the circulation is inevitably delayed or practically prevented. If we bury it deeply we not only get no good, but we may get harm by poisoning our wells and springs. It is the same with organic liquids. If these be poured on the surface, the 'living earth' (_i.e._ the humus stuffed with animal and microbial life) purges them of their organic matter, and transmits a relatively pure liquid to the deeper layers. If they be taken to the barren subsoil direct, as in underground sewers and cesspools, they escape the purifying action of air and aërobic organisms, and inevitably poison the water. Filthy liquids accumulating in cesspools and leaking _under pressure_ to our wells have cost us health and money incalculable.